Conventionally, cold-working steel bars and wire rods are mostly produced by cogging and rolling rimmed steel ingots. In some cases Al-killed or Al-Si-killed steel ingots are used for this purpose. All these steel ingots are produced by conventional ingot-making methods. It is, however, well known that better cold workability can be obtained with lower carbon contents, and in case of the same carbon content, rimmed steels are to be preferred over killed steels, while a lower sulfur content assures better workability. This means that the cold workability largely depends on the properties of the skin portion of the steel.
In continuous casting, however, when a rimmed steel is cast, a serious drawback resulting in an ultimate defect is that the generated gas remains near the surface portion of the cast billet or bloom, which produces blow-holes. Rolling of such billets or blooms results in considerably increased surface cracks. The problem of surface cracks due to blow-holes can be overcome by using killed steel which has been fully deoxidized with Al alone or with Al and Si, since the generation of gas within a mold can then be avoided. However, the presence of Al causes other problems instead in that a higher Al content in the melt tends to cause clogging of the tundish nozzle and forms aluminainclusion which remains in both the surface and the core portions of the billet or bloom, thereby deteriorating severely the quality of products rolled therefrom.
On the other hand, a higher silicon content in the steel melt tends to embrittle the steel and lowers the workability to a significant extent.
Continuous casting has an advantage in that segregation is greatly reduced due to factors such as the cooling rate, as compared with an ordinary ingot-making process. Thus, non-uniform distribution of constituents through various portions of a cast ingot or bloom is negligible.